Hydride transfer from 10-methyl-9,10-dihydroacridine (AcrH(2)) to 3,6-diphenyl-1,2,4,5-tetrazine (Ph(2)Tz), which contains a N=N double bond, occurs efficiently in the presence of Sc(OTf)(3) (OTf = OSO2-CF3) in deaerated acetonitrile (MeCN) at 298 K, whereas no reaction occurs in the absence of Sc3+. The observed second-order rate constant (k(obs)) increases with increasing Sc3+ concentration to approach a limited value. When AcrH(2) is replaced by the dideuterated compound (AcrD(2)), the rate of Sc3+-promoted hydride transfer exhibits the same primary kinetic isotope effect (k(H)/k(D) = 5.2 +/- 0.2), irrespective of Sc3+ concentration. Scandium ion also promotes an electron transfer from CoTPP (TPP2- = tetraphenylporphyrin dianion) and 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to Ph(2)Tz, whereas no electron transfer from CoTPP or (AcrH)(2) to Ph(2)Tz occurs in the absence of Sc3+. In each case, the observed second-order rate constant of electron transfer (k(et)) shows a first-order dependence on [Sc3+] at low concentrations and a second-order dependence at higher concentrations. Such dependence of ket on [Sc3+] is ascribed to formation of 1:1 and 1:2 complexes between Ph(2)Tz(.-) and Sc3+ at the low and high concentrations of Sc3+, respectively, which results in acceleration of the rate of electron transfer. The formation of 1:2 complex has been confirmed by the ESR spectrum in which the hyperfine structure is different from that of free Ph(2)Tz(.-). The 1:2 complex formation results in the saturated kinetic dependence of K-obs on [Sc3+] for the Sc3+-promoted hydride transfer, which proceeds via Sc3+-promoted electron transfer from AcrH(2) to Ph(2)Tz, followed by proton transfer from AcrH(2)(.+) to the 1:1 Ph(2)Tz(.-)-Sc3+ complex and the subsequent facile electron transfer from AcrH. to Ph(2)TzH(.). The effects of counteranions on the Sc3+-promoted electron transfer and hydride transfer reactions are also reported.